docs: def initialize_snapshot(self):
          """ Copy the DAG and validate """
          logger.debug('Initializing DAG snapshot for job {0}'.format(self.name))
          if self.snapshot is not None:
              logging.warn("Attempting to initialize DAG snapshot without " +
                           "first destroying old snapshot.")
  
          snapshot_to_validate = deepcopy(self.graph)
  
          is_valid, reason = self.validate(snapshot_to_validate)
          if not is_valid:
              raise DagobahError(reason)
  
          self.snapshot = snapshot_to_validate